Experimental Research On Realizing High Proportion Of Methanol Substitution And High Efficiency And Clean Combustion On The Heavy-duty Diesel Engine

The heavy-duty commercial vehicles which account for only 13.9% of the total automobile consumed 49.2% of China’s automobile fuel consumption. At the same time, the implementation time of national phase IV emission standard of the heavy-duty engine have been repeatedly postponed due to urea and fuel quality cannot be guaranteed in the national scope. Therefore, reducing the fuel consumption of heavyduty diesel engines and seeking new ways to achieve efficiency and clean combustion are of great significance. Diesel methanol compound combustion(DMCC) which can not only partially replace the consumption of diesel fuel, but also reduce nitrogen oxides(NOX) and particulate matter(PM) emissions, can well satisfy the both needs. But how to replace diesel with methanol at a higher proportion and how to meet the nation phase IV and above emission standard with the diesel methanol dual fuel(DMDF) combustion are the issues need to be studied urgently in front of us.For the purpose of realizing high proportion of methanol substitution and high efficiency and clean combustion on the DMCC heavy-duty diesel engine, two DMCC heavy-duty diesel engines were selected and the effects of methanol to diesel ratio, diesel main injection timing and pre-injection strategy of DMDF combustion on the combustion, performance and emission characteristics were studied in detail. Based on the above research, strategies of selecting the optimal substitution ratio of methanol(SRm), the limiting factors of SRm increasing, measures of further increasing SRm and strategies to achieve high efficiency and clean combustion were further discussed. And in this study, phenomenon and mechanism of premixed methanol auto-ignition, gas emissions, especially unconventional gas emissions of DMDF combustion and the emission level after coupling the simple after-treatment were also investigated in detail.The effects of increasing methanol to diesel ratio(RMD) on DMDF combustion, performance and emission characteristics were studied. The results showed that with the increase of RMD, the ignition delay under various loads was gradually prolonged and the THC and CO emissions increased rapidly. At high load, the brake thermal efficiency firstly increased and then decreased with the increase of RMD and the ringing intensity changed little. And at low load, they both declined continually. However, the ringing intensity was very sensitive to the RMD at medium load, it showed a slight increase at first and then increased quickly and it rapidly declined with the further increase of RMD. At high and low loads, the trade-off relationship of NOX and soot emissions was completely broken up and the higher the RMD was, the more advantageous to reduce NOX and soot emissions, while at medium load, the moderate RMD was more appropriate.The effects of diesel main injection timing on DMDF combustion, performance and emission characteristics were studied. The results showed that the maximum SRm under each load presented parabolic form when the diesel main injection timing was retarded from-8 to 4°CA ATDC. The maximum SRm was confined by pressure rise rate when the diesel main injection timing was earlier and it was confined by misfire when the diesel main injection timing was later. There was economic zone at DMDF combustion mode. The economical combustion could be achieved almost in the whole range of main injection timing and SRm at the medium and high loads and the fuel cost was low, while at low load the economical combustion only could be realized when the main injection timing was earlier and the SRm was not very high(less than 30%). At high load, by selecting higher SRm and main injection timings near the top dead center could get the best NOX and soot emissions. And at medium load, the main injection timing needed to be fixed at near the top dead center and the SRm needed to be calibration to get the best NOX and soot emissions. While at low load, retarding the main injection timing at pure diesel mode was a better way to get the best NOX and soot emissions. These results provide guidance for the calibration of DMCC heavy-duty diesel engine.The effects of pre-injection strategy on DMDF combustion, performance and emission characteristics were studied. The results showed that pre-injection strategy could increase the stability and SRm of DMDF combustion, the maximum SRm could exceed 80% at various diesel injection timings. At the same time, the pre-injection was able to expand the scope of economic combustion zone and make the zone at which the DMDF combustion could not run without pre-injection become economic or efficient combustion zone. However, pre-injection strategy was not very valid to reduce NOX and soot emissions, or even increased their emissions, it needed to adjust the diesel main injection timing and increase SRm for the further optimization of NOX and soot emissions. With the strategy of pre-injection and high SRm(more than 70%), NOX and soot emissions could be simultaneously reduced to a very low level with keeping the brake thermal efficiency high at the same time. Compared with the strategy of no preinjection, this strategy has a huge advantage on achieving the efficiency and clean combustion of DMDF.The effects of premixed methanol auto-ignition on DMDF combustion and engine performance were studied. The results showed that the auto-ignition of premixed methanol may be due to the duration of high temperature which is longer than the methanol ignition delay at that temperature. When the auto-ignition of premixed methanol occurred, the smoke emission increased remarkably. While the maximum incylinder pressure, maximum pressure rise rate and ringing intensity were mainly affected by SRm, and they all increased sharply at high SRm.The conventional and unconventional gas emissions and the emission level after coupling the simple after-treatment of the DMDF combustion were investigated. The results suggested that DMDF combustion would produce higher THC, CO, HCHO, NO2 and aromatic hydrocarbon emissions. DOC could not only basically eliminate the conventional exhaust pollutants such as THC and CO, but also had excellent elimination effect on the unconventional exhaust pollutants such as NO2, HCHO and aromatic hydrocarbons. Especially, the aromatic hydrocarbons could be completely eliminated by the DOC. The emission level of the heavy-duty diesel engine could be improved from national phase Ⅲ to Ⅳ by using DMDF combustion coupling with the simple after-treatment DOC&POC.